Pulse width modulation (PWM) is often used for controlling the powering of electrical loads. Examples of possible applications are the control of heating elements, or electric motors for fans or adjustment devices. A controller associated with an electrical load generates a periodic sequence of square-wave pulses whose width is modulated. A supply voltage is alternately switched on and off, wherein the so-called duty ratio d, defined as the ratio of the power-on time to the period duration, is such that on average a voltage d*Umax is present, where Umax is the peak value of the supply voltage. Conventional controllers are operated with an approximately constant PWM frequency, which may typically be in the range between approximately 10 kHz and 100 kHz.
It is known that driving electrical loads by PWM causes interference which is expressed, for example, as acoustic interfering noise during radio reception. Switching the voltage results in harmonics in the signal which may contain frequency components in the radio reception ranges. This may result in audible influences on the radio receiver, particularly in the medium wave band. These interferences are audible particularly when the harmonic is in the transmission bandwidth of the radio transmitter.
DE 10 2011 118 044 A1 describes a method for controlling an electrical load by means of pulse width modulation (PWM) in which for eliminating such interferences the switching frequency of the PWM is adapted to the previously determined carrier frequency of the radio transmitter for which interference is to be suppressed in such a way that an integer multiple of the switching frequency corresponds to this carrier frequency.
When a multiple of the switching frequency of the PWM, i.e., a harmonic from the harmonic spectrum of the switching frequency, matches the carrier frequency, ideally it is not audible since there is no audio signal present on the carrier frequency itself. The upper and lower adjacent harmonics thereof, which exactly match the carrier frequency, are outside the transmission bandwidth and therefore do not cause interference.
In this method, it is assumed, firstly, that the harmonic on the carrier frequency is small enough that it does not override the carrier, so that the latter is still detectable. Secondly, it is assumed that the harmonic is exactly on the carrier frequency. However, if the harmonic and carrier frequency are very similar but not identical, then this results in a beat between the frequencies, which may lead to new interferences.